Fatty acids are one of the principal components of lipids. In vivo, fatty acids are bonded to glycerin via an ester bond to form lipids (fats and oils) such as triacylglycerol. Further, many animals and plants also store and utilize fatty acids as an energy source. These fatty acids and lipids stored in animals and plants are widely utilized for food or industrial use.
For example, higher alcohol derivatives that are obtained by reducing higher fatty acids having approximately 12 to 18 carbon atoms are used as surfactants. Alkyl sulfuric acid ester salts, alkyl benzene sulfonic acid salts and the like are utilized as anionic surfactants. Further, polyoxyalkylene alkyl ethers, alkyl polyglycosides and the like are utilized as nonionic surfactants. These surfactants are used for detergents, disinfectants, or the like. Cationic surfactants such as alkylamine salts and mono- or dialkyl-quaternary amine salts, as other higher alcohol derivatives, are commonly used for fiber treatment agents, hair conditioning agents, disinfectants, or the like. Further, benzalkonium type quaternary ammonium salts are commonly used for disinfectants, antiseptics, or the like. Furthermore, lipids derived from plants are also used as raw materials of biodiesel fuels.
Fatty acids and lipids are widely used for various applications shown above, and therefore, it has been attempted to enhance the productivity of fatty acids or lipids in vivo by using plants and the like. Furthermore, the applications and usefulness of fatty acids depend on the number of carbon atoms. Therefore, controlling of the number of carbon atoms of the fatty acids, namely, a chain length thereof has also been attempted.
A fatty acid synthetic pathway of plants is localized in the chloroplast. In the chloroplast, an elongation reaction of the carbon chain is repeated starting from an acetyl-ACP (acyl-carrier-protein), and finally an acyl-ACP (a composite consisting of an acyl group being a fatty acid residue and an ACP) having 16 or 18 carbon atoms is synthesized. The synthesized acyl-ACP is formed into a free fatty acid by an acyl-ACP thioesterase (hereinafter, also simply referred to as “TE”). To the free fatty acid, CoA is bonded by an acyl-CoA synthetase. Then, the fatty acyl-CoA is incorporated into a glycerol skeleton by various acyltransferases, and is accumulated as the triacylglycerol (hereinafter, also simply referred to as “TAG”) formed in which three molecules of the fatty acids are ester-bonded with one molecule of glycerol.
In a biosynthesis of the TAG, first, bonding of an acyl group by a glycerol-3-phosphate acyltransferase (hereinafter, also referred to as “GPAT”) is caused in the sn-1 position of glycerol-3-phosphate (hereinafter, also referred to as “G3P”), and thus a lysophosphatidic acid (hereinafter, also referred to as “LPA”) is produced. Next, bonding of an acyl group by a lysophosphatidic acid acyltransferase (hereinafter, also referred to as “LPAAT”) is caused in the sn-2 position of the LPA, and thus a phosphatidic acid (hereinafter, also referred to as “PA”) is produced. Subsequently, dephosphorylation is caused by a phosphatidic acid phosphatase (hereinafter, also referred to as “PAP”), and thus diacylglycerol (hereinafter, also referred to as “DAG”) is produced. Finally, an acyl group is bonded therewith in the sn-3 position by a diacylglycerol acyltransferase (hereinafter, also referred to as “DGAT”), and thus the TAG is produced.
In addition, there is also a pathway in which the acyl group in a phospholipid is transformed into the DAG by a phospholipid:diacylglycerol acyltransferase (hereinafter, also referred to as “PDAT”) to produce the TAG.
Kinds of fatty acids to be bonded with the glycerol are wide-ranging, and various TAG compounds are formed depending on combinations of the fatty acids to be boded therewith. Then, in plants, the TAG compound is accumulated mainly in seeds or the like as an energy storage substance.
Methods of producing the TAG or the fatty acids have been so far proposed by using various acyltransferases.
For example, a method of producing TAG using Chlamydomonas reinhardtii in which DGAT2 is subjected to overexpression is proposed in Patent Literatures 1 and 2. In addition, Patent Literature 3 describes a DGAT derived from Nannochloropsis oculata, in which a possibility to be involved in a production of long-chain polyunsaturated fatty acids having 18 or 22 carbon atoms or the like is suggested. Moreover, a method of improving productivity of palmitic acid utilizing a GPAT derived from Thalassiosira pseudonana (see Patent Literature 4), a method of improving productivity of TAG utilizing a PDAT derived from Saccharomyces cerevisiae (see Patent Literature 5), a method of improving productivity of medium-chain fatty acids utilizing a LPAAT derived from Cocos nucifera (see Patent Literature 6), and the like are proposed.
Recently, algae attract attention due to its usefulness in biofuel production. The algae can produce lipids that can be used as the biodiesel fuels through photosynthesis, and do not compete with foods. Therefore, the algae attract attention as next-generation biomass resources. Moreover, it is also reported that the algae have higher lipid productivity and accumulation ability in comparison with plants. Research has started on a lipid synthesis and accumulation mechanism of the algae and lipid production technologies utilizing the mechanism, but unclear parts remain in many respects.